FIG. 1 shows in a simplified schematic diagram an ultrasonic sensor 12 positioned above a surface 16 of variable geometry of a metal body to be tested. The sensor 12 is supported a plastic wedge 11 so that it can send a projected sound beam 19 into the metal body below it at the desired angle to the surface 16. The projected sound beam 19 is part of an incoming sound beam 17 emanating from a piezoelectric element 13 within said ultrasonic sensor 12. Another part of said incoming sound beam 17 is reflected back at the contacting surface 14 of the wedge 11 as a scattered sound beam 18.
An ultrasonic sensor 12 of this type usually operates using the ultrasonic pulse-echo technique, in which a pulse of ultrasound (usually in the range 1 to 25 MHz) is projected into the material under test. When a defect is situated in the path of the sound beam, some of the sound energy (the “echo”) is reflected by the defect and returns to the sensor, with the travel time being used to give an indication of the distance of the defect from the sensor 12.
However, if the wedge 11 that supports the sensor 12 has to be machined with a fixed curvature, it is hard to fill the gap 15 between the contacting surface 14 of the plastic wedge 11 and a changing surface 16 with an acoustically compatible liquid. Thus the performance of the sensor arrangement 10 under these conditions is unpredictable.
The first attempt to improve the situation was (see FIG. 2) to manufacture a sensor arrangement 20 with a hollow housing 21 for the sensor 12, fill its inner space 22 with water and let the incoming sound beam 17 travel from the sensor 12 directly through the water into the material via surface 16. This was a definite improvement over the plastic wedge 11, but caused other problems, such as (i) the need to continuously pump water through the housing 21 to maintain the coupling and (ii) the ultrasonic noise generated by sound waves scattered from air bubbles 24 brought in by the water (scattered sound 25). So a solution had to be found to circumvent these difficulties.
In the prior art, document U.S. Pat. No. 3,550,438 discloses an ultrasonic inspection apparatus, which includes an electro-acoustic transducer, means for directing a column of water or other liquid normally to the surface of an article under inspection and at least one curved reflecting surface to reflect an ultrasonic beam emitted by the transducer and to concentrate the reflected beam on a region where the liquid column strikes the surface of the article. The apparatus is for inspecting a hot article and the liquid strikes the article at sufficient speed to prevent the liquid reaching boiling point where the column strikes the surface, while also acting as a coupling liquid. The apparatus is designed for inspection of a continuous cast billet of axisymmetric geometry, whereas the invention described here will inspect components of asymmetric geometry.
On the other hand, document U.S. Pat. No. 4,246,791, which relates to the different technical field of clinical diagnosis, discloses a portable ultrasonic scanning module, which includes a fluid-tight enclosure having a window at about the front thereof and a reflective scanner at about the rear thereof and generally facing the window. A transducer is mounted in the enclosure frontwardly of the reflective scanner with the ultrasound-emitting face of the transducer generally facing the reflective scanner and being oriented with respect to the reflective scanner at a relatively acute angle such that the beam effectively “doubles-back” past itself during its excursion through the scanning module. The module is used to produce images of the interior of the human body.
Document JP 10267903 describes a solution to reduce a multiecho and also to improve the accuracy of a flaw detection result in the inspection of a specimen with an ultrasonic wave, by forming an ultrasonic transfer part with polyethylene resin having an acoustic impedance of at most twice the acoustic impedance of an acoustic coupling medium. Within the disclosed solution water is used as an acoustic coupling medium. A longitudinal wave excited by an oscillator is propagated in a wedge, and reflected and refracted at a boundary surface. Then the transmitted longitudinal wave is propagated in the acoustic coupling medium. Where the ultrasonic wave is made incident from the water to acrylic resin, 37% of the sound pressure is reflected. In the case from the water to polyethylene resin, only 8.36% of the sound pressure is reflected, and therefore the level of the reflected wave is reduced.
Document EP 0 359 546 A2 discloses an ultrasonic scanning probe comprising a coupling fluid interposed between a transducer and a probe window, characterized in that said coupling fluid comprises a mixture of 1-Butanol and Glycerol. Document GB 2197 474 A discloses an acoustic borehole imaging tool with a lubrication fluid as internal fluid. Document U.S. Pat. No. 4,612,809 describes an ultrasonic probe which is used for medical diagnostic apparatus. The inner fluid is a fluorocarbon with a sound velocity of about one-third of the sound velocity of water.